'Nabla' setting for cloud texture
In order to calculate a normal, you can decide to take three samples in texture space. Typically that's the current sample location, and two other positions in texture U and V directions. This "nabla" value defines how far away these positions are. You will see how it works best on a high frequency procedural texture.

Typically tiny nabla values give minimal normal changes. Optimal is half the texture frequency (which you have to figure out by guessing unfortunately :).